1To open something (a lid, door, envelope), unseal or unpack it so the interior becomes accessible.
窓を開くと風が気持ちよく入ってきた。
When I opened the window, a pleasant breeze came in.
プレゼントの包みを開くと、本が入っていた。
When I opened the gift wrapping, there was a book inside.
古い封筒をそっと開いて手紙を取り出した。
I gently opened the old envelope and took out the letter.
2To bloom or unfold; to spread out (used for flowers, umbrellas, fans, etc.).
桜が一斉に開いて、町中が薄ピンク色になった。
The cherry blossoms all opened at once and the town turned light pink.
暗くなったので、傘を開いて家へ戻った。
It got dark, so I opened my umbrella and went home.
扇子を開いて風を送った。
I opened the folding fan and waved it to make a breeze.
3To open for business; to start operating (e.g., a store opening in the morning).
このカフェは朝7時に開くから、早朝に行けるよ。
This café opens at 7 a.m., so you can go early in the morning.
美術館は毎週火曜は閉館で、水曜に開く。
The art museum is closed every Tuesday and opens on Wednesday.
4To be wide or to widen (a gap, space, or difference increases).
棚と壁の間に小さな隙間が開いている。
There is a small gap between the shelf and the wall.
話が進むにつれて意見の差が開いてしまった。
As the discussion progressed, the difference of opinion widened.
裂け目がどんどん開いて危ない。
The tear keeps widening; it's dangerous.
5To hold or give (a meeting, party, ceremony, etc.); to organize an event.
来月、研究発表会を開く予定です。
We plan to hold a research presentation meeting next month.
会社が創立記念パーティーを開いた。
The company held a founding anniversary party.
町内会が夏祭りを開くことになった。
The neighborhood association decided to hold a summer festival.
6To found or establish (a nation, dynasty, sect, business); to start something new.
彼は京都に新しい茶道の流派を開いた。
He founded a new school of tea ceremony in Kyoto.
祖父は戦後まもなく小さな会社を開いた。
My grandfather opened a small company soon after the war.
7To open ports, borders, etc.; to make them accessible for trade or travel.
政府は新しい港を国際貿易のために開いた。
The government opened a new port for international trade.
戦後、いくつかの国境が順次開かれた。
After the war, several borders were gradually opened.
8To open an account (e.g., bank or brokerage account).
新しい銀行で口座を開いた。
I opened an account at a new bank.
オンラインで証券口座を開く手続きができる。
You can complete the process of opening a brokerage account online.
9To open up or clear new land or paths; to develop or pioneer (similar to 拓く).
入植者たちは森を切り開いて畑を開いた。
The settlers cleared the forest and opened fields.
新しいルートを開いて村への道が短くなった。
By opening a new route, the road to the village became shorter.
10To open (a file, window, program) in computing contexts.
画像ファイルをダブルクリックして開いてください。
Double-click the image file to open it.
ブラウザで新しいタブを開いて検索した。
I opened a new tab in the browser and searched.
11A mathematical usage meaning to expand an expression or to extract a root (specialized usage).
多項式を開くと、各項に分配される。
When you expand a polynomial, it is distributed into its terms.
12In cooking: to split and open a fish (hiraki) for cooking or filleting.
刺身用に鯵を開いて骨を取り除いた。
I opened (butterflied) the horse mackerel for sashimi and removed the bones.
魚を開いて塩をふり、干物にした。
I split the fish, salted it, and made dried fish.
13To convert kanji into hiragana (a typesetting or text-conversion use).
校正の際、難しい漢字は読みやすくするためにひらいて表記した。
During proofreading, difficult kanji were converted into hiragana to make them easier to read.
14To flare or billow (e.g., a skirt flaring out), to spread open in a flowing way.
風でスカートが開いてしまって恥ずかしかった。
My skirt was blown open by the wind and it was embarrassing.
選手のユニフォームの裾が試合中に開いた。
The hem of the player's uniform flared open during the match.
15In sports, to 'open up' — the body or shoulders move into a poor, open posture, causing loss of power or form.
バッティングで体が早く開くとボールに力が伝わらない。
If your body opens up too early in batting, power won't transfer to the ball.
スイング中に肩が開かないように注意しなさい。
Be careful not to let your shoulders open during the swing.
16A Go term (usually written in kana as ヒラく) meaning to extend or to open (a shape/position) on the board.
この形はヒラいて使うと厚みが出る。
If you use this shape to 'hiraku', it produces more thickness/strength.